Info I've read says the DCT is rated at 600Nm whereas the 6MT is only rated at 430Nm.

I know of a 135i in the UK where the gearbox gave up after 50k miles behind a remapped N55.

It's the reason BMW don't offer the manual trans bolted to the back of the 335d.

the 6MT's rating is down to the clutch rather than internals.. clutch stops being happy at more like 450ft-lb though rather than nm, the transmission has been run past 650ft-lb though, with no signs of wear.

the problem with DCT is the clutches can't really be upgraded easily as they're a lot more complex. soon though.